Even though Priestley was the first to publish an account of the isolation of oxygen, Scheele was the true discoverer of that gas. On Air and Fire was ready for the press in December 1775 but its publication was delayed for two years, in part because Scheele's friend Torbern Bergman did not deliver his promised preface until July 1777. In the meantime Priestley had published his discovery of oxygen gas, and other observations in Scheele's book had been made public, as well, much to Scheele's displeasure. Scheele's treatise contains accounts of the numerous chemical experiments Scheele performed to demonstrate that common air, when freed from "aerial acid" (carbon dioxide) and water vapor, consists of two gases: "fire air" (oxygen) which supports combustion, and "foul air" (nitrogen), which does not. Like Priestley, Scheele was an adherent of the phlogiston theory, and he accounted for oxygen's combustibility by describing it as peculiarly attractive to phlogiston. On Air and Fire also contains an account of Scheele's experiments with hydrogen sulfide gas, which he was the first to describe correctly and the first to synthesize. Scheele made numerous other important contributions to chemistry, including the discoveries of chlorine, manganese, and a number of acids. He also noted the action of light on chloride of silver and the insolubility of blackened silver chloride in ammonia - discoveries that would later prove significant for photography.